The invention relates to a spacer-electrolyte composite for electrolytic capacitors. More particularly, it relates to a porous film or membrane of controlled pore size which is filled with and contains the electrolyte. The composite may be a separate entity or may be cast on one or both sides of an electrode foil, preferably the cathode foil.
Typically, a wet liquid-type DC electrolytic capacitor includes at least one anode foil strip and at least one cathode foil strip separated by spacer strips, often paper, wound convolutely into a relatively tight roll. The spacer is impregnated with electrolyte, before or after rolling. In an AC capacitor, two anodes are used, separated by the spacer which is impregnated by electrolyte.
Spacers are used to keep the electrodes from directly contacting each other, and, when impregnated with electrolyte, they provide a conductive path. The more porous a spacer material is, the less resistance there will be to current flow through it. Similarly, the thinner a spacer is, the less resistance is offered. Low resistance is most desirable, particularly in AC capacitors, because of resistance heating. Various papers and plastic films have been used because they have the required tensile strength for winding even when thin. However, research has continued to find more porous materials so that series resistance may be reduced further. Such reduction by increase in porosity for porous polypropylene film vs. kraft paper is described by Ross et al in U.S. Pat. No. 3,908,157 issued Sept. 23, 1975. An ultramicroporous material made of cellulose triacetate and filled with liquid has been developed by Nichols and is described in U.S. Pat. No. 3,846,404 issued Nov. 5, 1974 and in U.S. Pat. No. 4,029,726 issued June 14, 1977, which also covers such structures made from specified cellulose nitrates, cellulose propionates, and cellulose acetate-cellulose nitrate mixtures. Details on the variation in properties with variation in composition and the exchange of the liquid phase are described in these patents.
These materials are described as polymer-liquid composites and are called Poroplastic.RTM., a registered trademark of Moleculon Research Corporation, Cambridge, MA.